1) Field of the Invention
The present invention relates to a steering control system for a self-propelled working vehicle comprising steerable front and rear wheels, direction detecting means for detecting a directional error of the vehicle relative to a reference direction, sideways detecting means for detecting a sideways error of the vehicle relative to a reference line extending in a vehicle running direction, and steering control means for steering the front and rear wheels in response to signals received from the direction detecting means and the sideways error detecting means.
2) Description of the Prior Art
This type of steering control system is disclosed in U.S. Pat. No. 4,573,547, for example. According to the known construction, when the facing direction of the vehicle is displaced from a reference direction, the directional displacement is corrected promptly by steering the front and rear wheels in opposite directions. When the position of the vehicle is displaced sideways from a reference position, the sideways displacement is corrected by steering the front and rear wheels to the same steering angle and in the same direction, namely steering the front and rear wheels in a parallel steering mode. Further, a proposal is made in this prior patent to correct the directional displacement and the sideways displacement simultaneously. The correction is carried out by offsetting, from the steering angle of the front and rear wheels in the parallel steering mode for correcting the sideways displacement, only the front wheels by a predetermined angle in a direction to correct the directional displacement.
In the proposed method of correcting both the directional displacement and sideways displacement by offsetting only the front wheels from the steering angle in the parallel steering mode, however, the sideways displacement is corrected after the directional displacement or vice versa depending on directions in which these errors have taken place. This correcting process cannot be very smooth and may adversely affect results of the operation by the working vehicle.